Method for removing compounds containing sulfur from fuels

ABSTRACT

The present invention relates to a process for removing sulfur-comprising compounds from fuels, wherein a sulfur-comprising fuel is brought into contact with copper-1,3,5-benzenetricarboxylic acid MOF.

The present invention relates to a process for removing sulfur-comprising compounds from fuels, wherein a sulfur-comprising fuel is brought into contact with copper-1,3,5-benzenetricarboxylic acid MOF.

The desulfurization of motor fuel is usually carried out in refineries using large-scale industrial processes in the production of the fuels. Apart from extraction processes, thermal, catalytic and hydrodesulfurization processes are also employed. Furthermore, catalytic processes based on microbiological processes are used. The first processes are carried out at high temperatures and pressures, so that they can be realized on board a motor vehicle only with a great safety outlay, if at all.

The reduction of the sulfur content of fuels is of great industrial interest both for meeting legal requirements and also with regard to the sulfur compatibility of exhaust gas after-treatment systems and fuel cells. A legal boundary condition for reducing the sulfur content of fuels is given by the reduction in the sulfur content to 50 mg/kg of sulfur sought for 2005 within the European Union. In addition, an additional 1.5 ct per liter of mineral oil tax has been imposed on all fuels having a sulfur content above 10 mg/kg since Jan. 1, 2003 in Germany. A new regulation even provides for the compulsory introduction of sulfur-free fuels from Jan. 1, 2009 and thus intends to implement the directives of the European Parliament and the Council regarding the quality of spark-ignition and diesel fuels. “Sulfur-free” in this sense means that a fuel can have a sulfur content of up to 10 mg/kg.

Sulfur-free fuel or highly desulfurized fuel is necessary so as not to poison exhaust gas after-treatment catalysts such as NOx storage catalysts or oxidation catalysts. In the case of an internal combustion engine operated using an excess of air (e.g. “lean-burn engine” or diesel engine), a large quantity of nitrogen oxides is formed due to the principle of the engine. A possible exhaust gas purification concept therefore provides for introduction of an NOx storage catalyst which can store nitrogen oxides for some time into the exhaust gas train of a motor vehicle. This “storage phase”, in which the catalyst is “filled” with the exhaust gas component to be stored, is followed by a desorption phase in which the catalyst is “emptied”. Alkaline earth metal salts or alkali metal salts are used as nitrate storage media. Unfortunately, such compounds react preferentially with the sulfur oxides which are likewise present in the exhaust gas and are formed during combustion of the sulfur compounds present in the fuel to form alkali metal sulfates or alkaline earth metal sulfates. After a certain period of operation, which depends on the concentration of the sulfur compounds in the fuel, such storage catalysts therefore lose their storage capability. They have to be regenerated (also referred to as “desulfated” or “desulfurized”). Processes by means of which such storage catalysts can be desulfurized are described, for example, in EP 858 837, EP 860 595 or in EP 899 430. Here, the engines are no longer operated in the normal above-described alternating lean/rich operation during the regeneration phase but are instead continually operated rich and a particular minimum temperature which is required for desulfurization has to be achieved. This process requires an increased fuel consumption. Furthermore, the NOx storage catalyst ages with each desulfurization operation.

A reduced-sulfur fuel accordingly leads to better efficiency of the vehicle since no fuel-consuming desulfurization procedures have to be carried out. In addition, the life of the catalysts is increased.

Analogously, sulfur-free fuel or highly desulfurized fuel increases the life of fuel cells, since sulfur poisons the catalysts immobilized on the electrodes of the fuel cells, just as in the case of the exhaust gas after-treatment catalysts.

Low-sulfur fuel is also employed in diesel engines where the particle emission in the exhaust gas can be reduced by reducing the sulfur content of the diesel fuel.

DE 198 45 397 describes a process for desulfurizing motor fuels on board a motor vehicle. Adsorption media disclosed are, in particular, solids such as Al, Mg, Si or Ti in oxidic form. Examples are Al₂O₃, MgO, SiO₂, TiO₂, zeolites, hydrotalcites or mixed oxides.

EP 303 882 describes the removal of hydrogen sulfide by means of transition metal carboxylates. Mention is made by way of example of carboxylates of titanium, vanadium, chromium, iron, cobalt, nickel, copper, zinc and/or manganese. Particular preference is given to carboxylates of divalent or trivalent iron or divalent manganese.

In Abstracts of Papers, 235th ACS National Meeting and in the abstract for their presentation on Jul. 14, 2008 at the AICHE 2008 Annual Meeting in Philadelphia, Thompson et al. discloses that functionalized metal-organic frameworks are promising adsorbents for the absorption of organic sulfur compounds. MOFs (metal-organic frameworks) are promising storage media because of their high BET surface area, but the MOFs do not have a high affinity for organosulfur compounds. To increase this affinity, an MOF-5 (Zn-terephthalic acid MOF) support was doped with molybdenum (3% by weight) or MoC_(x) (10% by weight). A surface area of greater than 2000 m²/g was achieved. The absorption of dibenzothiophene in a feed comprising 35 ppmw_(s) was 2.5 mg of S/g_(sorbent).

WO 2005/63354 describes a process for decreasing the content of sulfur and/or sulfur-comprising compounds in biochemically produced organic compounds such as bioethanol. As adsorbents, preference is given to using zeolites, but metal-organic frameworks can also be used. However, no specific MOF compounds are mentioned; furthermore, no examples using MOFs are disclosed.

WO 2008/21194 describes the use of metal-organic frameworks for desulfurizing liquids, where the metal-organic frameworks are composed of chains of octahedral metal oxides which are linked via aromatic dicarboxylic acids. As possible metals, mention is made of aluminum, vanadium, chromium, iron, titanium, zirconium, hafnium and cerium.

WO 2006/125739 discloses the use of a suspension comprising a metal-organic framework for reducing odor. Odorous substances are, for example, sulfur or sulfur-comprising compounds.

WO 2006/122920 describes the removal of odorous substances from gases using metal-organic frameworks. Gases mentioned are, inter alia, sulfur-comprising compounds.

EP 1 702 925 discloses the suitability of a porous metal-organic framework for the absorption of unintentionally spilt liquids such as disinfectant, an odorous substance, inorganic or organic solvent, fuel, brake fluid or oil.

Owing to the disadvantages indicated, there continues to be interest in increasing the efficiency of the desulfurization of fuels. An increase in efficiency can be achieved (i) by a greater desulfurization capacity, (ii) by a kinetically optimized desulfurization, so that the residence times are shorter and higher space-time yields are achieved, and/or (iii) by a greater desulfurization intensity. Owing to the disadvantages indicated above for any residual sulfur content, item (iii) is a particular focal point in industry.

It is accordingly an object of the present invention to increase the efficiency of low-temperature processes for desulfurizing fuels compared to the prior art. In particular, the process should be suitable for use at atmospheric pressure in mobile systems. In addition, the process should offer not only desulfurization of conventional fuels but also a further lowering of the sulfur content of fuels which are already low in sulfur.

The systems for desulfurization should also take up only a small installation volume in the motor vehicle and add little additional weight and should ideally be able to be integrated as directly as possible into the fuel supply system. The maintenance of the desulfurization unit should be compatible with conventional maintenance intervals for the motor vehicle.

We have surprisingly found a process for removing sulfur-comprising compounds from fuels, which has a good efficiency when using an MOF compound from the prior art. In the process of the invention, a sulfur-comprising fuel is brought into contact with copper-1,3,5-benzenetricarboxylic acid MOF.

The process of the invention is suitable for all commercial, motor fuels, in particular gasoline, diesel, heating oil, kerosene and/or methanol.

The process of the invention is particularly suitable for the further desulfurization of already highly desulfurized spark-ignition and diesel fuels. The previously desulfurized spark-ignition and diesel fuels typically still have a residual sulfur content of from 8 to 15 mg/kg.

The sulfur-comprising compounds to be removed from the fuels are typically thiophene, carbon disulfide, hydrogen sulfide, thioethers and/or thioesters.

The process of the invention is advantageously carried out at a temperature of from 0 to 100° C. and a pressure of from 0.5 to 5 bar. The process of the invention is particularly preferably carried out at ambient temperature and ambient pressure, so that the process can advantageously be carried out on board a mobile system without further introduction of energy.

The sulfur uptake is advantageously largely concluded within a reaction time, i.e. a residence time, of from 5 to 100 minutes, preferably from 5 to 60 minutes, in particular from 45 to 60 minutes.

Possible mobile systems are, in particular, motor vehicles and also railroad trains, aircraft and ships.

Copper-1,3,5-benzenetricarboxylic acid MOF is advantageously used in a concentration of from 5 mg/ml to 200 mg/ml, preferably from 10 to 100 mg/ml, in particular from 10 to 50 mg/ml, based on a sulfur content in the fuel of from 8 mg/kg to 27 mg/kg.

The sulfur content of the fuel is advantageously reduced by means of the process of the invention by at least 30%, preferably at least 40%, particularly preferably at least 50% and in particular at least 60%, based on the initial sulfur content.

The sulfur content of the fuel is, at a typical initial sulfur content of from 5 mg/kg to 50 mg/kg, reduced to a residual sulfur content of from 2 mg/kg to 10 mg/kg by means of the process of the invention.

The sulfur content of the fuel is advantageously reduced by from 10 to 30 mg/kg, preferably from 15 to 30 mg/kg, in particular from 20 to 30 mg/kg.

The sulfur uptake per gram of copper-1,3,5-benzenetricarboxylic acid MOF is advantageously at least 0.2 mg/g, preferably at least 0.25 mg/g; for example, the sulfur uptake is from 0.2 mg/g to 1 mg/g, preferably from 0.25 to 0.75 mg/g.

In the case of the previously desulfurized fuels having a sulfur content of from 8 to 15 mg/kg, a further reduction of the sulfur content by advantageously a further 1 to 5 mg/kg to a value of from 5 mg/kg to 10 mg/kg is achieved by means of the process of the invention. In this case, the sulfur uptake per gram of copper-1,3,5-benzenetricarboxylic acid MOF is advantageously from 20 μg/g to 700 μg/g. The sulfur content of the previously desulfurized fuels is thus advantageously reduced by a further at least 5%, preferably at least 10%, particularly preferably at least 15% and in particular at least 20%, by means of the process of the invention. The sulfur content of the previously desulfurized fuels is advantageously reduced by from 10 to 25% by means of the process of the invention.

Copper-1,3,5-benzenetricarboxylic acid MOF is generally known to those skilled in the art and is described, for example, in J. Mater. Chem. 2006, 16, 626-636. Copper-1,3,5-benzenetricarboxylic acid MOF is now commercially available. Apart from the conventional method of producing metal-organic frameworks, these can also be prepared by an electrochemical route. In this respect, reference may be made to DE 103 55 087 and EP 1 687 462. The metal-organic frameworks prepared in this way have particularly good properties in respect of the adsorption and desorption of chemical substances, in particular gases. They thus differ from those prepared in a conventional way, even when the latter are formed from the same organic and metal ion constituents and can therefore be considered to be new frameworks. For the purposes of the present invention, electrochemically prepared metal-organic frameworks are particularly preferred.

The copper-1,3,5-benzenetricarboxylic acid MOF can optionally be used in admixture with other MOFs. Furthermore, all auxiliaries and additives known to those skilled in the art can be added to the copper-1,3,5-benzenetricarboxylic acid MOF.

The copper-1,3,5-benzenetricarboxylic acid MOF can be used in powder form or as shaped bodies. There are essentially no restrictions with regard to the possible geometries of these copper-1,3,5-benzenetricarboxylic acid MOF shaped bodies. For example, possibilities are, inter alia, pellets such as disk-shaped pellets, pills, spheres, granules, extrudates such as rods, honeycombs, grids or hollow bodies. The shaped bodies can be produced by all methods known to those skilled in the art, as described, for example, in DE 10 2005 012 087 on page 20 ff.

The copper-1,3,5-benzenetricarboxylic acid MOF can also advantageously be applied to a support in order to ensure optimal accessibility of all active surfaces of the MOF. Possible supports are all supports known to those skilled in the art, for example supports based on aluminum oxide, ceramic supports (e.g. cordierite), metallic supports (e.g. steel sheet honeycomb bodies or high-temperature aluminum-chromium sheet) or polymeric supports.

The process can advantageously be used directly on board a mobile system, in particular a motor vehicle, and can thus reduce the sulfur content of commercial fuels in-situ or limit it to a prescribed maximum level.

Copper-1,3,5-benzenetricarboxylic acid MOF can advantageously be structurally integrated into the fuel filter. There, the copper-1,3,5-benzenetricarboxylic acid MOF can, for example, be coated with the filter material or be arranged directly upstream or downstream of the filter material.

Furthermore, desulfurization of the fuel on entry into the vehicle tank of the motor vehicle is possible. Inclusion of the copper-1,3,5-benzenetricarboxylic acid MOF in a container which is impermeable to the copper-1,3,5-benzenetricarboxylic acid MOF but is permeable to the fuel and can be integrated into the vehicle tank is likewise according to the invention.

Furthermore, the copper-1,3,5-benzenetricarboxylic acid MOF can advantageously be used as a recyclable one-use device which can be introduced, for example by the user, before filling of the tank and can be obtained, for example, at the filling station.

The process of the invention is preferably carried out continuously so that the desulfurized fuel is fed without intermediate storage to the engine.

It is also possible for a sulfur sensor which monitors the sulfur content of the motor vehicle to be integrated on board the motor vehicle. Integration of the amount of fuel consumed enables the amount of sulfur introduced into the catalyst to be determined. The point in time at which desulfurization is necessary can thus be calculated precisely.

A further application of the low-sulfur fuel is use in the desulfating of a catalyst in the exhaust gas after-treatment system of an engine.

Furthermore, the low-sulfur fuel can also be used as reducing agent for deNOx catalysts in lean-burn exhaust gas.

The advantage of the present invention for the desulfurization of fuels is that the lower-sulfur or sulfur-free fuel is available in the fuel container and can thus be supplied immediately when the engine is started. Use of the sulfur-reduced fuel obtained enables the life of the exhaust gas after-treatment systems to be increased significantly.

In addition, the use of a low-sulfur fuel makes it possible for the user or the automobile industry to achieve further legal requirements inexpensively.

The invention is illustrated below with reference to figures and with the aid of examples. In the figures:

FIG. 1 shows an experimental set-up for determining the adsorbent properties FIGS. 2 a and 2 b show the dependence of the desulfurization of a model oil on the loading of the adsorber container with adsorption medium

FIG. 3 shows the desulfurization of model oils by addition of copper-1,3,5-benzenetricarboxylic acid MOF (Cu-BTC-MOF) as a function of time

FIG. 4 shows the desulfurization of a real low-sulfur fuel

FIG. 5 shows the dependence of the desulfurization on the MOF used

FIG. 6 to FIG. 9 each show advantageous constructions for carrying out the process of the invention.

FIG. 1 shows an experimental set-up for determining the adsorbent properties of the Cu-BTC-MOF:

A particular concentration of the adsorbent 2 was weighed into a closed container 1 filled with fuel or model oil. Uniform contact with the entire surface area of the adsorbent was ensured by the continuous, circular motion of a vortexer 3. After a fixed sampling time, the fuel was taken from the container and purified via two filter units 4-5 so that no residues of the adsorbent remained in the filtrate and the desulfurized fuel could be passed directly to analysis 6. For a quantitative analysis, the filtrate was fed off-line to an elemental analyzer. The filtration residue, i.e. the adsorbent enriched with sulfur components, was dried and was then available for solid-state analysis or regeneration experiments.

FIG. 2 shows the dependence of the desulfurization of a thiophene-comprising model oil on the loading of the adsorber container with adsorbent:

The model oil comprised a thiophene-comprising dodecane solution having a total sulfur content of 27 mg/kg. The use of 12.5 mg/ml or 50 mg/ml of Cu-BTC-MOF enabled an adsorbent concentration-dependent sulfur reduction to 16.2 mg/kg or 7.7 mg/kg to be achieved. Here, the sulfur uptake was 0.67 mg/g for 12.5 mg/ml of Cu-BTC-MOF and 0.29 mg/g for 50 mg/ml of Cu-BTC-MOF (FIG. 2 a).

The model oil comprised a thiophene-comprising dodecane solution having a total sulfur content of 31 mg/kg. The use of 50, 100 and 200 mg/ml of Cu-BTC-MOF enabled an adsorbent concentration-dependent sulfur reduction to 13, 9 and 7 mg/kg, respectively, to be achieved. Here, the sulfur uptake was 0.4, 0.2 and 0.1 mg/g for 50, 100 and 200 mg/ml, respectively, of Cu-BTC-MOF (FIG. 2 b).

FIG. 3 and FIG. 4 show, in measurements over time, the sulfur reduction in a thiophene-comprising model oil and a real previously low-sulfur fuel as a function of time:

The measurements were carried out as described for FIG. 1. In the desulfurization of the thiophene-comprising model oil (FIG. 3), the total sulfur content was able to be reduced by 62% from 27 mg/kg to 10.2 mg/kg within the first hour by use of 50 mg/ml of Cu-BTC-MOF. In the following hours, the sulfur content remained virtually constant. The desulfurization proceeded rapidly and is completely finished after only one hour.

Even highly desulfurized fuel can be desulfurized further by use of Cu-BTC-MOF. As FIG. 4 shows, the 8 mg/kg present could be reduced by 22% to 6.3 mg/kg. The rapid sulfur uptake by the Cu-BTC-MOF in the first hour can likewise be seen clearly.

FIG. 5 shows a comparison with the prior art and further MOF compounds.

The measurements were carried out as described for FIG. 1. Firstly a Zn-terephthalic acid MOF, the support material selected by Thompson et al., and secondly Cu-DABCO-terephthalic acid MOF and Cu-isophthalic acid MOF were selected as MOF compounds. The model oil comprised a thiophene-comprising dodecane solution having a total sulfur content of 31 mg/kg. The MOF compounds were each used in an amount of 50 mg/ml (6.6% by weight).

FIG. 5 shows that Zn-terephthalic acid MOF and Cu-DABCO-terephthalic acid MOF display no sulfur uptake. Cu-isophthalic acid MOF shows only a small sulfur uptake. When Cu-Isophthalic acid MOF was used, the sulfur content could be reduced only to 25.5 mg/kg, corresponding to a removal performance of 0.05 mg/g (m_(s)/m_(MOF)). In contrast, when Cu-BTC-MOF was used, the sulfur content could be reduced to 11 mg/kg, corresponding to a removal performance of 0.28 mg/g (m_(s)/m_(MOF)).

Furthermore, it was found that the nitrogen content of the sample is significantly increased when using Zn-terephthalic acid MOF and Cu-DABCO-terephthalic acid MOF.

FIGS. 6 to 9 show sketches of advantageous constructions for the possible use of the adsorbent directly on board the motor vehicle:

In FIG. 6, the adsorbent 7 is integrated into the fuel filter 8 and arranged in series downstream of the filter material 9. An electric fuel pump 10 pumps the fuel from the tank 11 through the adsorber-filter unit 8 before it is injected into the engine 12. Various alternatives are available for construction of the adsorber-filter unit. In FIG. 6 A, the adsorbent 7 is arranged in a volume-filling manner downstream of the filter material, so that replacement of only one unit would be possible. To reduce weight, construction in the form of a separation column as shown in FIG. 6 B would also be conceivable. Here, the fuel flows firstly into the interior of the adsorber-filter unit 8 and goes from there via a permeable layer of filter material 9 and adsorbent 7 into an outer region before being injected into the engine.

In FIG. 7, the adsorbent 13 is integrated into a framework structure 14 which is permeable for the fuel directly in the fuel tank 15. The framework structure can, for example, comprise plates which are coated with the adsorbent, as can be seen in FIG. 7 A. The plates can be taken individually from the fuel tank in order to replace them. In FIG. 7 B, the adsorbent is present in a cartridge which can be taken from the tank when required. The advantage of this construction is the on-average significantly longer contact time of the adsorbent with the fuel to be desulfurized.

In FIG. 8, the adsorbent 16 is accommodated in a preliminary tank 17 which is filled during the tank filling operation. After desulfurization, the fuel goes into the main tank 18 in which an optional sulfur sensor 19 monitors the instantaneous sulfur content.

A similar construction is shown in FIG. 9, but here the adsorbent 20 is present in a container which is arranged as “after-tank” 22 downstream of the actual fuel tank 21. An electric fuel pump 23 feeds desulfurized fuel from the after-tank to the engine. 

1. A process for removing at least one sulfur-comprising compound from at least one fuel, the process comprising: bringing the at least one sulfur-comprising fuel into contact with copper-1,3,5-benzenetricarboxylic acid MOF.
 2. The process according to claim 1, wherein the copper-1,3,5-benzenetricarboxylic acid MOF is present in a concentration of from 5 mg/ml to 100 mg/ml.
 3. The process according to claim 1, wherein a sulfur content of the at least one fuel is reduced to from 2 mg/kg to 10 mg/kg.
 4. The process according to claim 1, wherein sulfur uptake by the copper-1,3,5-benzenetricarboxylic acid MOF is at least 0.2 mg/g.
 5. The process according to claim 1, wherein a sulfur content of at least one previously sulfur-reduced fuel having an initial sulfur content of from 8 to 15 mg/kg is reduced by at least a further 10%.
 6. The process according to claim 1, wherein an uptake of sulfur is concluded within a reaction time of from 5 minutes to 60 minutes.
 7. The process according to claim 1, carried out at ambient temperature and ambient pressure on board a mobile system.
 8. The process according to claim 1, wherein the at least one fuel is selected from the group consisting of petrol, diesel, heating oil, kerosene, and methanol.
 9. The process according to claim 1, wherein copper-1,3,5-benzenetricarboxylic acid MOF is structurally integrated into a fuel filter or into a vehicle tank of a mobile system or is a separate device which can be introduced before filling of the tank.
 10. The process according to claim 1, carried out continuously.
 11. The process according to claim 1, wherein a low-sulfur fuel obtained is employed in desulfating of an exhaust gas after-treatment system of an engine or as reducing agent for deNOx catalysts in lean-burn exhaust gas.
 12. The process according to claim 2, wherein a sulfur content of the at least one fuel is reduced to from 2 mg/kg to 10 mg/kg.
 13. The process according to claim 2, wherein sulfur uptake by the copper-1,3,5-benzenetricarboxylic acid MOF is at least 0.2 mg/g.
 14. The process according to claim 3, wherein sulfur uptake by the copper-1,3,5-benzenetricarboxylic acid MOF is at least 0.2 mg/g.
 15. The process according to claim 2, wherein a sulfur content of at least one previously sulfur-reduced fuel having an initial sulfur content of from 8 to 15 mg/kg is reduced by at least a further 10%.
 16. The process according to claim 3, wherein a sulfur content of at least one previously sulfur-reduced fuel having an initial sulfur content of from 8 to 15 mg/kg is reduced by at least a further 10%.
 17. The process according to claim 4, wherein a sulfur content of at least one previously sulfur-reduced fuel having an initial sulfur content of from 8 to 15 mg/kg is reduced by at least a further 10%.
 18. The process according to claim 2, wherein an uptake of sulfur is concluded within a reaction time of from 5 minutes to 60 minutes.
 19. The process according to claim 3, wherein an uptake of sulfur is concluded within a reaction time of from 5 minutes to 60 minutes.
 20. The process according to claim 12, wherein an uptake of sulfur is concluded within a reaction time of from 5 minutes to 60 minutes. 